Cadillac Repair: open fuel pump circuit, oil pressure switch, fuel pump relay

Question
hello i have a 1981 eldo with the wonderful V8-6-4  i was driving and it started studdering and had very little power.. i got home, the car idled bad then quit and wouldnt start again.. this was about 3 yrs ago.. i havent had much time to play around wiht it since.. i recently checked the ecc codes and it was  20 which is a open fuel pump circuit. the does not get gas in the injectors.. i replaced the fuel pump and still nothing.  ive poored gas down the injectors and the car would start but wouldnt idle long... any idea's what could be causing a open pump circuit?  any ideas would be appreciated

Answer
As soon as you turn ON the ignition switch to ON position, Just before you turn the key to the start position when the engine starts to crank over, A 2 second battery voltage signal is sent from the computer to the fuel pump relay and then from the relay back to the fuel pump in the fuel tank. This runs the fuel pump for ONLY 2 seconds of which pressurizes the fuel system in preparation to start the engine.

Once the engine starts, The computer BYPASSES the fuel pump relay and  sends constant contionous battery voltage from the computer to the oil pressure switch of which is now CLOSED because the oil pressure is UP because the engine is running. Therefore the battery voltage flows thru the oil pressure switch back to the fuel pump to run the fuel pump constantly until you turn OFF the engine.
Therefore if your POSITIVE you are hearing that 2 second pump run just when you turn the ignition switch to the ON position ONLY, just before you turn the ignition key to the start position, I would have to say  you have a weak fuel pump upon initial start up.
If you are NOT hearing that 2 second fuel pump run, Then I would say that the fuel pump relay is defective.



tom